The present invention relates to a pressure relief door and particularly to a pressure relief door that can respond to both positive and negative pressures in a heating, ventilation, and air-conditioning (HVAC) system. Such systems include one or more fans for moving air and ductwork through which the air moves. Dampers are also provided in such systems to automatically close in response to fire or smoke.
The function of a pressure relief door is to prevent ductwork from imploding or exploding in the event dampers close while the fan is still operating. Opening outward or inward at a specified pressure setting, the present invention permits rapid neutralization of the pressure differential between the inside and outside of the HVAC system.
HVAC duct damage due to instantaneous closure of dampers is a potential problem and concern of design engineers and contractors. Laboratory tests prove that extreme pressures occur upstream and downstream of instantaneously closing dampers. FIG. 4 shows a possible scenario. Assume there is a run of duct similar to that shown. If a damper L closes for some unexpected reason with the system in operation, the damper L effectively stops the passage of air through the damper.
A moving column of air has mass and therefore has inertia. Thus, the air immediately downstream of the damper will continue to move after the damper closes. Since no air can pass through the closed damper to replace this moving column of air, a void, or partial vacuum, is left in the duct. The kinetic energy of this moving column of air is a function of air velocity. At low air velocities, there would not be enough kinetic energy developed to be a problem, but as velocity increases, a tremendous amount of energy will be developed which can create a problem in large duct diameters.
Two ways have been used to protect ducts from collapse in this situation. Some manufacturers of dampers have developed devices that prevent the instantaneous closing of dampers, giving the duct a chance to adapt to the changed condition. However, delaying the closing of a damper can cause severe results in the case of a fire, for example. The other method of protection is to install a pressure relief door in the ductwork downstream from the damper, so that the pressure relief door will open to relieve the partial vacuum when the partial vacuum reaches a previously set point. However, such previous devices have not been easily pre-set at the factory.
Explosion relief panels and doors have been used previously to relieve excess pressure within a system due to an explosion, but such panels have generally not also been usable as negative pressure relief doors.
There is a need for a pressure relief door that can be installed in a duct to open inwardly or outwardly in response, respectively, to excess negative or positive pressure in the duct. The door must be easily pre-set at the factory to open in response to a particular pressure.